1. Field of the Invention
The present invention relates to an air-intake system of an engine, and more particularly to an air-intake system of an engine mounted in a small watercraft such as a personal watercraft (PWC).
2. Description of the Related Art
In general, an air-intake system of a four-cycle engine is configured to have an ambient-air inlet, an air cleaner for cleaning ambient air taken in from outside through the inlet, a throttle body for adjusting an air-intake amount, an air-intake box for temporarily storing the air (in the case of a multi-cylinder engine), and an air-intake pipe for leading the air into an air-intake port of the engine. In this air-intake system, these components are connected in this order.
Meanwhile, in a four-cycle engine, an internal pressure of a crankcase varies according to reciprocation of pistons or the like. Accordingly, in order to reduce an increased internal pressure of the crankcase, a breather pipe is provided between the crankcase and the air cleaner.
A blow-by gas flows from a combustion chamber of the engine into the crankcase through a clearance between a cylinder and a piston. The blow-by gas is delivered into the air cleaner through the breather pipe and mixed into the air inside the air cleaner. The blow-by gas mixed into the air is delivered from the air cleaner into the air-intake box, and further into the combustion chamber through the air-intake pipe connected to the air-intake box. The blow-by gas drawn into the combustion chamber is combusted with the air.
Typically, an oil pan is provided below the crankcase. Lubricating oil is reserved in the oil pan. The oil is sometimes changed into mist, which is mixed with the blow-by gas. In order to liquefy the oil mist mixed with the blow-by gas and return the oil into the oil pan, an oil separator is provided at a position in the breather pipe.
Such oil mist is delivered together with the blow-by gas into the air cleaner through the breather pipe and into the air-intake box. The oil mist delivered into the air-intake box is drawn into the combustion chamber of the engine together with the air and the blow-by gas through the air-intake pipe.
However, part of the oil mist delivered into the air-intake box is liquefied inside the air-intake box and is stored in a bottom portion thereof.
In order to remove the oil residing in the air-intake box in the four-cycle engine configured as described above, it is necessary to provide an oil discharge hole on the bottom portion of the air-intake box and discharge the oil residing in the air-intake box through the oil discharge hole on a regular basis.
However, such regular oil discharge is burdensome. In particular, in a small watercraft such as a personal watercraft, the oil discharge is difficult because of its limited inner space.
The present invention addresses the above described condition, and an object of the present invention is to provide an air-intake system of an engine which is capable of automatically delivering oil residing inside an air-intake box into a combustion chamber according to an operation of an engine.
According to the present invention, there is provided an air-intake system of an engine, comprising an air-intake port provided in a cylinder head; an air-intake box provided in an air-intake flow passage of the engine; an air-intake pipe forming part of the air-intake flow passage connecting the air-intake box to the air-intake port; and a connecting pipe provided between the air-intake box and the air-intake pipe to allow the air-intake box and the air-intake pipe to communicate with each other, wherein the connecting pipe is configured such that one opening end portion thereof is connected to a bottom portion of the air-intake box to open inside the air-intake box and the other end portion thereof is connected to the air-intake pipe to open inside of the air-intake pipe.
A pressure difference is generated between the one opening end portion and the other opening end portion of the connecting pipe due to the flow rate (speed) difference of air taken in into the engine. Specifically, since the flow rate of the taken-in air flowing inside the air-intake pipe is higher than that of the air flowing inside the air-intake box, an internal pressure of the air-intake pipe is lower than an internal pressure of the air-intake box during an operation of the engine. In the above-described air-intake system, because of the pressure difference between the opening end portions of the connecting pipe, oil separated from the taken-in air with a blow-by gas inside the air-intake box and reserved in the bottom portion thereof, is suctioned up from the one opening end portion of the connecting pipe and drawn into the air-intake pipe through the other opening end portion. The oil is drawn into the combustion chamber together with the air. Thereby, in the air-intake system provided with the air-intake box at a position in the airflow passage, it is not necessary to discharge the oil residing inside the air-intake box regularly.
Preferably, the engine has multiple cylinders, and a plurality of air-intake ports and air-intake pipes, the air-intake pipes are configured to connect the plurality of air-intake ports to the air-intake box, respectively, and at least two of the air-intake pipes are connected to the air-intake box through the connecting pipe to allow an inside of the air-intake pipes and an inside of the air-intake box to communicate with each other. In this system, the oil reserved in the bottom portion of the air-intake box is quickly suctioned up through the connecting pipes.
Preferably, the air-intake box is configured to have an inner bottom face thereof inclined such that a portion of the inner bottom face which is close to a position where the air-intake box is connected to the one opening end portion of the connecting pipe is lower. In this structure, the oil reserved in the bottom portion of the air-intake box is easily guided to the one opening end portion of the connecting pipe and larger amount of oil is suctioned up from the bottom portion of the air-intake box through the connecting pipe.
Preferably, the air-intake system further comprises a one-way valve provided in the branch portions of the connecting pipe, and the one-way valve is configured to permit flow of a fluid from the one opening end portion toward the other opening end portion inside the connecting pipe and not to permit flow of the fluid from the other opening end portion toward the one opening end portion inside the connecting pipe. In this structure, backflow of the oil suctioned up through the one-way valve is inhibited. Consequently, the suctioned oil can be reliably delivered into the air-intake pipe.
Preferably, a direction in which the other opening end portion of the connecting pipe opens substantially corresponds with an air flow direction in which taken-in air flows inside the air-intake pipe. In this structure, a static pressure of the other opening end portion of the connecting pipe becomes lowest and, correspondingly, the pressure difference between the one opening end portion and the other opening end portion of the connecting pipe becomes larger. By utilizing this pressure difference, the oil can be efficiently suctioned up through the connecting pipe.
Preferably, the air-intake pipe includes a curved portion for allowing the air flow direction inside the air-intake pipe to be curved, and the other opening end portion of the connecting pipe is connected to an outer side of the curved portion of the air-intake pipe.
In the case of a curved flow passage, a flow rate of a fluid flowing inside the flow passage is higher and a pressure of the fluid is lower on the outer side of the curved flow passage. In the above structure, the pressure difference between the one opening end portion and the other opening end portion of the connecting pipe becomes larger, which facilitates upward suction of the oil.
Preferably, the other opening end portion of the connecting pipe is connected to the air-intake pipe in the vicinity of the air-intake port. In this system, the oil drawn from the connecting pipe into the air-intake pipe can be delivered into the combustion chamber more reliably. In addition, it is possible to minimize contamination of an inner wall of the air-intake pipe by the oil adhering to the inner wall.
The above and further objects and features of the invention will more fully be apparent from the following detailed description with accompanying drawings.